This invention relates to projection television and more particularly to an improved envelope for a projection television tube, to a projection television tube incorporating the envelope and to a process for forming the envelope.
Projection television is a well established technique. However, because of the high cost involved in manufacturing the projection television tubes used, projection television has generally been limited to expensive, complicated, large screen units which are both difficult to install and expensive to maintain in satisfactory alignment. Thus the complexity and cost of the presently available projection television tubes have critically curtailed the development and acceptance of projection television systems for home entertainment, a use which could represent a very high-volume market.
Projection television systems for color projection commonly include three tubes each having a different color (conveniently referred to as red, green and blue) projecting pictures which are superimposed in registry on a common view screen. Typically, the most efficient of such tubes incorporate the optics of a Schmidt-type projection system and comprise a target illuminated in a single color by a suitable specific phosphor as an electron beam raster, a spherical reflector directing the light around the periphery of the target and a correction lens for any spherical aberration.
Within these tubes, certain dimensions are critical, a fact which has heretofore given rise to the high cost of constructing the projection tube envelopes to achieve a high level of reliability and reproducibility of optical characteristics. In particular, it is essential to locate and maintain the spacing between the mirror surface and the target surface at a predetermined value within about .+-.0.001 to 0.002 inch (about 0.025 to 0.050 mm). Projection tubes of the prior art have therefore been constructed to incorporate expensive and complicated means both to position the target during envelope and tube manufacture and to adjust its location both during manufacture and installation. A relatively early approach to achieving a partial solution to this problem was to incorporate the mirror within the tube envelope by forming it on that internal envelope surface opposing the target surface. This, however, still necessitated maintaining the critical dimensions of more than one envelope component as well as the precise relative positioning of the components. (See for example U.S. Pat. Nos. 2,467,462 and 2,637,829.) As this art has progressed in its development, more and more complex mechanisms have been incorporated into the tubing envelope and associated with it to achieve and maintain the required mirror/target alignment and critical distance.
Thus, it is obvious that it would be highly desirable to provide an envelope for a projection television tube which is relatively simple in construction but which is capable of achieving during construction and maintaining during use the required precise spatial distance and optical alignment between mirror and target surfaces.